1. Field of the Invention
The present invention relates to a rotor shaft sealing method and structure of an oil-free rotary compressor such as a tooth type rotary compressor, whose sealing structure can prevent lubrication oil of the drive mechanism of the rotor from leaking into the compression chamber of the compressor even when the pressure of the compression chamber becomes lower than atmospheric pressure, which occurs under some operation condition of the compressor.
2. Description of the Related Art
Generally, a tooth type rotary compressor consists of two rotors, a male rotor and a female rotor, each having claw-like teeth, or lobes. The rotors turn in opposite directions without contact to each other to compress gas trapped in the compression pockets formed between the lobes and inner surface of a compressor casing as the rotors rotate. As the rotors do not contact with each other and with the inner surface of the compressor casing, the rotors do not wear and have a long life. Further, lubrication of the rotors is not needed because of non-contact engagement of the rotors, and clean compressed gas not contaminated with lubricant can be obtained. Compression ratio obtained by this type of compressor is relatively low, and required high compression ratio is obtained with high efficiency in many cases by composing a two-stage compressor unit comprised of a lower pressure stage compressor and a higher pressure stage compressor connected in series and driven separately. Working of the tooth type compressor will be explained hereunder referring to FIG. 6a to FIG. 6d 
In FIG. 6a, a male rotor 02 having claw-like lobes engages with a female rotor 03 having claw-like lobes with very tight clearances in a compressor housing 01. Gas g to be compressed is sucked from a suction opening 04 into the compressing chamber as the rotors 02 and 03 rotate in directions indicated by arrows. In FIG. 6b, the suction opening 04 is closed by the rotors 02, 03, and the sucked gas g is confined in a pocket surrounding the lobes of the female rotor 03 and in a pocket surrounding the lobes of the male rotor 02. The rotors convey the gases confined, or trapped in the pockets from the suction side to the pressure side as shown in FIG. 6c, where the pockets are communicated and the volume of the sum of the two pockets reduces as the rotors rotate and the gases are compressed until the female rotor 03 uncovers the discharge port 05. In FIG. 6d, the discharge port 05 is uncovered by the female rotor 03 and the compressed gas c between the rotors is discharged through the discharge port 05.
It is necessary requirement for an oil-free rotary compressor such as an oil-free tooth type compressor that lubrication oil for lubricating rotor shaft bearings is prevented from leaking into the compression chamber of the compressor in order to supply clean compressed gas not containing the lubrication oil. Positive pressure is produced in the compression chamber in load operation of the compressor, but when the compressor is operated under no load, pressure in the compression chamber becomes negative, for the upstream side of the suction port of the compressor is shut by a suction closing mechanism. When pressure in the compression chamber becomes negative, intrusion of lubrication oil supplied to the rotor bearing into the compression chamber through the shaft seal may occur.
Rotor shaft sealing structure of a screw compressor type supercharger is disclosed in Japanese Laid-Open Utility Model Application No. 3-110138 (hereinafter “JP 3-110138”). The sealing structure is composed such that a lip seal (contact seal) and a non-contact seal are located between rotor shaft bearing and the compression chamber, an airspace is formed between both the seals, a communicating passage is provided to allow the airspace to communicate with outside air, and a check valve is provided in the communicating passage to allow outside air to be sucked into the airspace when negative pressure is produced in the airspace.
With the above-described construction, pressure difference between the compression chamber and the airspace is reduced through the non-contact seal having fin-like annular protrusions such as a labyrinth seal. When pressure in the compression chamber is positive, higher than atmospheric pressure, escaping of the positive pressure air in the compression chamber passing through the communicating passage is prevented by the check valve closed by positive pressure in the communicating passage, and when pressure in the compression chamber is negative, the check valve is opened by negative pressure in the communicating passage and outside air is sucked into the air space, thus the airspace serves as a pressure equalizer room. In this way, intrusion of the lubrication oil into the compression chamber is prevented by maintaining the airspace not lower in pressure than that in the bearing part.
A rotor shaft sealing structure disclosed in Japanese Laid-Open Patent Application No. 7-317553 (hereinafter “JP 7-317553”) relates also to shaft sealing structure of a screw compressor type supercharger. The shaft sealing structure is composed such that a contact seal (lip seal, for example) for sealing lubrication oil lubricating the rotor shaft bearing and a pressure fluctuation alleviating member (a piston ring movable in axial direction, for example) are located between rotor shaft bearing and the compression chamber, an airspace which serves as a pressure equalizer room is formed between the contact seal and the pressure fluctuation alleviating member, and a communicating passage opened into outside of the compressor.
However, with the sealing structure disclosed in the JP 3-110138, in a case where leakage of lubrication oil occurs from the bearing part to the airspace through the lip seal, oil leaked to the airspace is difficult to escape outside because of the presence of the check valve in the communicating passage. When pressure in the compression chamber becomes negative while the leaked lubrication oil is present in the airspace, the lubrication oil residing in the airspace is apt to be ingested into the compression chamber.
Further, in a case where the communicating passage is clogged for any reason, the leaked lubrication oil accumulates in the airspace without being allowed to escape outside, and the leaked lubrication oil accumulated in the airspace is easily ingested into the compression chamber when negative pressure is produced in the compression chamber.
According to the sealing structure disclosed in the JP 7-317553, the communicating passage for communicating the airspace surrounding the rotor shaft to the outside of the compressor is not provided with a check valve. However, a means for allowing lubrication oil leaked into the airspace to escape outside in a convincing way is also not disclosed in JP 7-317553. Further, a means for allowing lubrication oil accumulated in the airspace when the communicating passage is clogged to escape outside is not disclosed in either JP 3-110138 or JP 7-317553. Further, in the above references, the rotor shaft sealing structure is composed such that atmospheric air can be introduced into the airspace as a pressure equalized room, however, sealing effect will be increased by introducing air pressurized to a pressure higher than atmospheric pressure to the pressure equalized room.